In view of the recent tendency to speed up the photographic processing of photographic light-sensitive materials, a so-called water-resistant photographic paper utilizing a polyolefin-coated paper as a support has been developed and put to practical use. These water-resistant photographic papers using as a support a paper substrate, both surfaces of which are coated with a polyolefin, a hydrophobic resin, have advantages in that fatigue of the developer can be reduced and the time required for rinsing and drying after development can be greatly shortened, since the support absorbs the developer to a lesser extent. Furthermore, these supports have superior dimensional stability.
However, when compared with photographic images formed in conventional photographic paper materials using a baryta-coated paper as a support, photographic images formed in photographic paper materials utilizing a polyolefin-coated paper as a support have the disadvantage that the sharpness is poor.
The cause of the foregoing defect is believed to be due to the light-barrier effect of the white pigment incorporated in a polyolefin layer of the support, this polyolefin layer being in contact with a light-sensitive silver halide emulsion layer, is obtained only insufficiently since the amount and weight% of the white pigment are small. Therefore, light is scattered or diffused in the polyolefin layer. However, it is very difficult to increase the weight% of the white pigment in the polyolefin layer to about 68% or more.
Furthermore, when photographic images formed in the conventional photographic paper materials using baryta-coated paper as a support are compared, the photographic images formed in the photographic paper materials utilizing a polyolefin-coated paper as a support have the disadvantage that the image density contrast looks as if it were deteriorated depending on observation conditions. For example, when photographic images on polyolefin-coated paper are looked at, a clear distinction can be observed between the case where they are placed on a white plate and the case where they are placed on a black plate. That is, when the photographic images are placed on the black plate, the entire image looks as if it were blackened, and this is more noticeable in areas with a white background. As a result, it looks as if the image contrast drops and the degree of whiteness of the white background is deteriorated. On the other hand, with photographic images on baryta-coated paper, the above-described phenomenon does not occur or occurs only to a limited extent. Similarly, when photographic images on polyolefin-coated paper are observed not on plates as described above but while they are held in the air, their image contrast looks as if it was reduced, in comparison with photographic images on baryta-coated paper.
The cause of the phenomenon is believed to be due to the light-barrier effect of white pigment incorporated in the polyolefin layer of the support, this polyolefin layer being in contact with a light-sensitive silver halide emulsion layer. This effect insufficiently obtained since the amount and weight% of the white pigment are small. Therefore, part of light is allowed to pass through the support.
In order to improve the reduction in sharpness of the above-described defects, Japanese Patent Application (OPI) No. 54225/82 (the term "OPI" as used herein means a "published unexamined Japanese patent application) discloses a method in which a hydrophilic colloid layer containing a white pigment and a dye capable of being decolored during the process of development is sandwiched between a polyolefin-coated paper substrate and a light-sensitive silver halide emulsion layer. In one of the examples described in the specification of Japanese Patent Application (OPI) No. 64235/82, a hydrophilic colloid layer is provided which contains a white pigment in an amount of about 48% expressed in terms of weight%. Even at these low contents, the sharpness is improved to a certain extent, as described in this patent specification, if the white pigment is used in combination with the dye. That is, if the white pigment content is small, the mean distance between the pigment grains is increased and the degree of permeation of light through the white pigment-containing layer, i.e., the degree of diffusion of light, is increased due to the synergistic effects because of the effect that the amount of light capable of passing in a straight line through spaces between the grains and the effect that the distance for which light can advance in a straight line is greatly increased. On the other hand, when the white pigment content is high, in particular, as the theoretical closest packing rate (in the case of completely spherical grains, about 90%, expressed in terms of weight%) is reached, the degree of diffusion of light in the white pigment-containing layer is decreased abruptly and the sharpness of the photographic images is greatly increased. In practice, however, the white pigment grains are not spherical and grain sizes are not indential. Moreover, the white pigment grains are not always uniformly dispersed in the hydrophilic colloid. It has been found experimentally that when the white pigment content of the white pigment-containing layer exceeds about 68% by weight, the sharpness is greatly increased, although this does not completely correspond to the theoretical closest packing rate. It has further been found that the photographic sensitivity of the light-sensitive silver halide emulsion layer is increased although only to a small extent.
The reason for this is believed to be due to the degree of permeation of light into the white pigment-containing layer being reduced, i.e., the amount of light passing through to the support is reduced, the proportion of light returning to the light-sensitive silver halide emulsion layer is increased.